I've been continuing my research into this issue and have found this.
Is it related to the problem I am having?

Code:

2007-10-04 10:32:31,709 WARN [LmtpServer-118] [] ParsedMessage - exception while analyzing message; message will be partially indexed
java.lang.NullPointerException
at com.zimbra.cs.mime.MimeHandler$HandlerInfo.getInstance(MimeHandler.java:401)
at com.zimbra.cs.mime.MimeHandler.getMimeHandler(MimeHandler.java:117)
at com.zimbra.cs.mime.ParsedMessage.analyzePart(ParsedMessage.java:587)
at com.zimbra.cs.mime.ParsedMessage.analyzeMessage(ParsedMessage.java:532)
at com.zimbra.cs.mime.ParsedMessage.analyze(ParsedMessage.java:252)
at com.zimbra.cs.lmtpserver.ZimbraLmtpBackend.deliverMessageToLocalMailboxes(ZimbraLmtpBackend.java:249)
at com.zimbra.cs.lmtpserver.ZimbraLmtpBackend.deliver(ZimbraLmtpBackend.java:122)
at com.zimbra.cs.lmtpserver.LmtpHandler.doDATA(LmtpHandler.java:420)
at com.zimbra.cs.lmtpserver.LmtpHandler.processCommand(LmtpHandler.java:197)
at com.zimbra.cs.tcpserver.ProtocolHandler.processConnection(ProtocolHandler.java:231)
at com.zimbra.cs.tcpserver.ProtocolHandler.run(ProtocolHandler.java:198)
at EDU.oswego.cs.dl.util.concurrent.PooledExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Thread.java:619)